This invention concerns the field of power supply systems for telecommunications applications and, in particular, to power supply systems for generating ringing voltage signals in remotely located telecommunications units.
2. Prior Art Systems and Methods
In a conventional telephone network, a wire pair extends from a "central office" location to a subscriber's premises. Battery voltage is typically applied to the wire pair at the central office to power the subscriber's telephone service set(s), and an AC voltage, traditionally a nominal 90 volts at 20 Hertz, is applied to "ring" the service set(s) at the subscriber's premise, in order to signal an incoming call and induce the subscriber to remove the handset from its cradle, creating an "off hook" condition. The standard telephone service set within the traditional U.S. telephone system includes an audible call annunciator, classically a bell, more recently a piezoelectric resonator, which is broadly resonant at a given "ringing frequency," broadly in a range of 16 to 67 hertz, wherein, for example, 20 hertz is a typical ringing frequency used in the U.S. and 25 hertz is a typical ringing frequency used in Europe.
Ringing voltage is generated at a central office in a number of ways. For example, a rotary generator, or ringing machine, may be employed consisting of a single-speed motor, either AC or DC, depending upon the local power supply. The motor rotates one or more AC generators which generate the desired ringing frequencies and voltages. Magnetic generators operating from AC power mains at the central office have also been used to generate ringing signals. Such generators employ resistors, transformers and tuned circuits of inductors and capacitors in order to develop the desired ringing signal. Vibrating reed converters have also been employed to generate ringing voltage from a 48 volt (DC) central office battery supply. These converters typically include two magnetic coils, an armature and a reed assembly mounted on a frame, and convert the 48 volts DC into an AC square wave of the desired frequency for ringing. A device such as a mechanical interrupter is typically employed to divide the ringing generator's signal into alternating ringing and silent periods, the traditional U.S. standard being alternating periods of two seconds "ringing" and four seconds "silent" For example, one implementation has been a motor for rotating a shaft carrying a number of cams which operate switch contacts that switch the ringing signal on and off. All of the foregoing approaches are generally bulky, however, and require significant amounts of primary operating power.
Fiber optic networks are proliferating within U.S. and foreign telecommunications networks, wherein communication signals are sent via lightwave signal transmission over an optical medium, typically an "optical fiber", instead of via electrical signal transmission over a metallic medium, typically a wire pair. Known fiber optic telecommunications distribution networks typically comprise a plurality of optical fibers extending from a central office location to a remotely located subscriber interface unit (SIU), wherein each optical fiber comprises a communications path for one or more subscribers served from the SIU. At the SIU, the optical signals are converted to electrical signals and then transmitted the remaining distance to each subscriber over a traditional wire pair. Because centralized signal transmission for several subscribers over an optical fiber is often more cost effective than individual signal transmission over a wire pair, it is desirable to be able to locate each SIU as close as possible to the subscribers it serves.
It is not possible, however, to send central office battery power and ringing voltage signals via optical transmission. Thus, operating power and ringing signal generation must be provided locally at each SIU and delivered to subscribers in an appearance functionally identical to the existing "wire plant" network discussed above. For example, U.S. Pat. No. 5,321,596, entitled "DC/DC/AC Power Supply For A Subscriber Interface Unit" issued to D. Hurst and assigned to the assignee of the present application, discloses an efficient and compact power supply system for generating on-demand ringing voltage signals in an SIU. The Hurst system includes a DC to DC switched mode power supply, which converts a source of DC primary power into DC operating power for an SIU. A ringing voltage signal is derived by converting a reference sine wave into an AC voltage signal derived from a DC to AC power converter. In one described embodiment, the ringing voltage signal is a 56 volts-RMS sine wave centered at -48 volts. The ringing voltage signal is centered at -48 volts in order to provide sufficient "offset" voltage from ground, which normally requires an offset of at least -40 volts, so that the SIU can detect an off-hook condition on a particular subscriber line receiving the ringing signal.
Most functions of an SIU that consume operating power, (e.g., optical/electrical signal conversion, off-hook monitoring, call processing, etc.), require a substantially constant operating power level, P.sub.SIU =V.sub.SIU *I.sub.SIU, where V.sub.SIU represents available operating voltage supplied to the SIU, and I.sub.SIU represents the operating current load of the SIU. If the SIU operating power level falls below a minimum threshold for any non-negligible duration of time, (e.g., for more than 10 ms), the SIU will cease operation until the available operating power level recovers, thereby causing undesirable service interruptions and call failures. Because the required operating power is constant, a drop in the operating voltage level will cause the SIU to attempt to pull additional current in order to maintain minimum threshold power. There are limits, however, to the SIU's ability to increase current in order to maintain minimum operational power because of the corresponding additional drain on the operating voltage supply.
In particular, as demonstrated in FIG. 1, when the SIU current load is less than the current load at maximum power, the relationship between the operating voltage level V.sub.SIU and current load I.sub.SIU is linear and stable. However, when the current load exceeds the current load at maximum power P.sub.MAX, the operating voltage becomes unstable and can drop precipitously. Thus, for stable operation of an SIU, a certain, minimum voltage supply, V.sub.MIN, must be maintained in order to prevent the total operating power from dropping below the required threshold.
Operating voltage for an SIU is typically supplied over a "feeder" line from a voltage source. In some instances, the voltage source is local to the SIU. In many cases, however, the feeder line extends over a substantial distance, thereby significantly reducing the available voltage supply at the SIU location because of the resistance of the feeder line (R.sub.FEEDER in FIG. 1). Because the SIU voltage supply is directly related to the feeder line resistance R, feeder line distances must often be kept shorter than is otherwise desirable, (e.g., no longer than where R =R.sub.MAX in FIG. 1), and can represent a limiting restraint on the distance an SIU can be located from a central office. Further, SIU operating cost constraints typically require that power supplied to an SIU be limited to that actually necessary to meet applicable service levels--i.e., sufficient to ensure that call completion and call processing functions are fully operational under most all, non-extraordinary, operating conditions.
Voltage used for ringing signals imposes a relatively significant transient load on the operating power of an SIU, e.g., typically up to 4 watts per line. In particular, simultaneous ringing of two or more lines can cause a severe impact on the operating voltage supply of an SIU, even if the overlap is only momentary. For illustrative purposes, FIG. 2 depicts the incremental impact of three subscriber lines, A, B, and C, respectively, on the current load and corresponding operating voltage of an SIU, where subscriber lines A, B and C each require simultaneous ringing signal generation. In particular, during each interval T.sub.IMAX, where all three subscriber lines require ringing voltage generation, the operating voltage level falls below V.sub.MIN, Creating an "undervoltage" condition.
Built into traditional telecommunications industry service level criteria is the likelihood of a very small percentage of subscriber lines in an SIU requiring simultaneous ringing voltage generation during a particular time interval. However, the relatively large corresponding operating voltage supply required to ensure that sufficient stand-by power is available for accommodating simultaneous on-demand ringing signal generation for more than a very small percentage of subscriber lines would impose a prohibitively high operating cost. As a result, the operating voltage supply of an SIU, V.sub.SIU, is susceptible to low, or "undervoltage" conditions, if simultaneous ringing voltage is required for more than a small percentage of subscriber lines, i.e., if the number of subscriber lines simultaneously requiring ringing voltage exceeds the capability of the power supply. Other factors, such as, for example, fluctuating input voltage levels and/or feeder line resistance may further cause the operating voltage supply of an SIU to be particularly susceptible to an undervoltage condition, even when only a small number of subscribers require simultaneous ringing voltage generation.
Thus, it is desirable to provide a power supply system for generating on-demand ringing voltage signals in an SIU, or the like, wherein corrective steps are taken during a low voltage condition to limit power consumed by ringing voltage generation, in order to ensure that sufficient continuous operating power is delivered to the SIU and uninterrupted service maintained, without imposing significant stand-by power supply costs.